Microvalves are the subject of continuing research. Microvalves generally utilize microelectromechanical systems (MEMS) technology to control fluid flow in microfluidic systems. Microvalves have been variously used in chemical analysis, micro-total analysis system (μTAS), gas/liquid sample injection, mixing, lab-on-a chip, micropumps and compressors, and so on.
U.S. Pat. No. 6,148,635, for example, discloses a compact active vapor compression cycle heat transfer device. The device of the '635 patent includes a flexible diaphragm serving as the compressive member in a layered compressor. The compressor is stimulated by capacitive electrical action and drives the relatively small refrigerant charge for the device that is under high pressure through a closed loop defined by the compressor, an evaporator and a condenser. The evaporator and condenser include microchannel heat exchange elements to respectively draw heat from an atmosphere on a cool side of the device and expel heat into an atmosphere on a hot side of the device. The overall structure and size of the device is similar to microelectronic packages, and it may be combined to operate with similar devices in useful arrays. The '635 patent makes use of passive microvalves, e.g., flap microvalves and active electrostatic microvalves in the heat transfer device to direct fluid flow in the closed loop in one direction. In this invention the microvalves simply hold off the fluid flow until a desired high pressure is reached and then they rapidly open. They cannot close against the higher pressures or be switched on and off at any time desired, nor can they bi-directionally route the fluid flow. The active electrostatic microvalves are used simply to hold-off the opening of the microvalve for the pressure to reach higher values. Other types of devices require active microvalves that can be arbitrarily switched in time and can reroute fluid flows.
Active microvalves include an actuator that responds to application of electrical energy, whereas passive microvalves do not. Active microvalves have an important advantage over passive microvalves, in that their fluidic resistances can be changed with respect to time and applied pressure by an applied control voltage or current. Also, an active microvalve can operate in resistance to fluid pressure. On the other hand, passive microvalves are typically smaller and are often easier to fabricate than known active microvalves. Passive microvalves can open rapidly, even as fast as microseconds. Active microvalves, however, take milliseconds or much longer to open or close, particularly if switching high pressures.
Different actuation principles have been used in active microvalves. Actuators that have been tested in active microvalves include solenoid plungers, piezoelectric actuators, electromagnetic actuators, shape memory alloys, pneumatic actuators, bimetallic actuators, and thermopneumatic actuators. The last four types can potentially switch relatively high pressures, but tend to be slow or very slow. Electrostatic actuators have also been investigated due to the ability to scale well as size shrinks and due to potentially very high switching speeds, but with less success. Comb-drive electrostatic actuators have been investigated, but occupy a significant amount of space relative to the overall size of the microvalve, particularly if actuating high pressures. In a comb-drive, the generating electrostatic force is limited due to the inverse proportionality of the force to the gap between the electrodes. Additionally, electrostatic microvalves that employ in-plane actuators, such as comb drives, are ill-suited for out-of-plane flow geometries. In-plane designs have limited applications.
Known electrostatic actuators often require relatively high applied voltages (>100 V) to generate sufficient force to open and close the microvalves against even a modest pressure (0.1 atm) since the electrostatic force is inversely proportional to the square quadratic of gap distance between electrodes, if operated in planar mode, and is proportional to electrode area over seal area if operated in comb drive mode. Known electrostatic microvalves also exhibit a binary open or closed operation, with little ability to operate at positions between fully open and fully closed to adjust flow rates for a given pressure. In addition, normally closed (or fail-closed) electrostatic microvalves have proven difficult to achieve. Typical known designs do not open against a pressure but rather act with applied pressure (i.e., the microvalve seat is pressurized acting to push the microvalve open). Such known electrostatic actuated microvalves tend to be leaky, with relatively high back flows (order of 0.1% or greater with respect to forward flows) possible.
Additionally, known electrostatically actuated MEMS microvalves typically employ silicon-based architectures, with doped silicon as the conductor and silicon oxide or nitride as the material of the seats and valves. This creates relatively hard microvalves and seats, which also have difficulty sealing at the interface and can suffer from wear during operation. Other issues with such microvalve seats include hydrogen bonded sticking (“stiction”) problems when humid gases or aqueous liquids are valved, which reduces the reliability of the device.
The issue of discrete flow control from open and closed states has also recently been addressed by developing electrostatic actuator arrays for more precise control of the microflow. See, Collier et al. “Development of a Rapid-Response Flow-Control System Using MEMS Microvalve Arrays,” J. of MEMS, Vol. 13, No. 6, December 2004, pp. 912-922. To address the issue of relatively high voltage operation of electrostatic devices used to apply high forces, touch-mode actuation has been developed in order to increase the electrostatic force without needing voltages well over 100 V.
One type of touch-mode actuation device that has been proposed uses an unmovable electrode surface shaped in a smooth curve for the other moving electrode to touch with these electrodes continuously, such that the moving electrode is pulled in on actuation. Legtenberg, et al., “Electrostatic Curved Electrode Actuators,” J. of MEMS, Vol. 6, No. 3, September 1997, pp. 257-265; Li, et al, “DRIE-Fabricated Curved Electrode Zipping Actuators with Low Pull-in Voltage,” Transducers 03, 2003, pp. 480-483.
Touch-mode actuation generates electrostatic force between the two touching electrodes, which are separated by one or more dielectric layers that prevent electrical shorting and arcing. Achieving high force at reasonable voltage, e.g., less that 100V, requires that the gap between the electrodes be very small, since the magnitude of the electrostatic force is proportional to the square of the electric field. Minimizing the electrode gap competes with other practical difficulties, however, as exemplified by the prior research discussed in the background of this application. One such issue is dielectric breakdown. In the closed position of a touch-mode capacitance microvalve, the spacing between the electrodes is determined solely by the thickness of dielectric separating the electrodes. Ideally, the dielectric thickness would be minimal to increase the electrostatic force generated upon application of voltage to drive the electrodes away from each other. With very thin dielectric layers, e.g., less than a few microns and down to one micron, the electric field becomes too high for typical dielectric materials to sustain. For example, if 100 V is applied across 1 micron, the field is 100 V/micron or 1 megavolt per centimeter, which is very high for typical dielectric materials to sustain. Dielectric breakdown, of course, produces device breakdown.
Another type of touch-mode actuation device that has been proposed involves attaching one and the other ends of the moving electrode to the upper electrode and lower electrode, respectively for the moving electrode to zip with one electrode and to unzip the other electrode, which makes the moving electrode s-shaped. Fluidic capacitance caused by the curved electrode, and longer traveling path of the s-shaped electrode can degrade the microvalve response time. See, Sato, et al. “An Electrostatically Actuated Gas Microvalve with an S-Shaped Film Element,” J. of Micromech. & Microeng., Vol. 4, 1994, pp. 205-209; Shikid et al. “Response Rime Measurement of Electrostatic S-Shaped Film Actuator Related to Environmental Gas Pressure Conditions,” Proc. of IEEE MEMS, 1996; Oberhammer, J., and G. Stemme, “Design and fabrication aspects of an S-Shaped film actuator based DC to RF MEMS switch,” J. of MEMS, Vol. 13., No. 3, June 2004, pp. 421-428. Complicated curves and shapes present considerable fabrication hurdles, however.
A normally closed flat membrane touch-mode capacitance microvalve that acts out of plane has also been investigated. See, Philpott, et al., “Switchable Electrostatic Micro-Valves with High Hold-off Pressure,” 2000 Solid-State Sensors and Actuators Workshop, Hilton Head Island, S.C., Jun. 4-8, 2000, p. 226-229. This type of microvalve was demonstrated to be able to hold off very high pressures (>18 atm) applied to the microvalve seat without opening or leaking, and had no measurable reverse leakage or flow. However, the microvalve was not able to close against high pressures (only on order of 1 atm or less), nor could it be opened against a reverse pressure applied to the side opposite the microvalve seat.
A rolling action electrostatically actuated microvalve has been proposed to reduce required actuation voltage. See, U.S. Pat. Nos. 6,968,862 and 6,837,476. In these devices, a diaphragm including an electrode is provided in a space between opposing walls. One of the opposing walls is curved and includes an electrode that is attached to the wall and follows its curved shape. Fluid pressure is also maintained on both side of the diaphragm to reduce the pressure differential and the required actuation voltage. In the '862 patent, the curved shape is to make the diaphragm actuate in a rolling action. This causes the diaphragm to effectively squeeze the fluid out from between the diaphragm and its touch interface with the curved electrode. The curve creates a continuous gradient in the separation distance between the diaphragm and the stationary electrode, and this results in the rolling action that reduces actuation voltage. An embodiment includes a third electrode on the other opposing wall, which has a microvalve seat and is flat. Due to the curve in the upper electrode, the third electrode is at a considerable gap from the diaphragm over a substantial region. The gap acts to increase the time needed for actuation, as well as reduces the pressures over which the microvalve can open and close or switch directions of flow.
Similarly, the touch-mode capacitance systems that use two smooth surfaces, where the diaphragm forms an “S” shape, are designed to make the diaphragm smoothly move from one position the next. If there is a sharp disruption, or jump, in the surface that leaves a gap, the high electrostatic contact force is lost or is greatly diminished, since the electrostatic force created by a given voltage is proportional inverse of the square of the separation distance (distance between the electrode plus dielectric thickness on the membrane). Thus, for example, doubling the distance between the electrodes by creating a gap (the gap is zero when touching, and the force is determined only by the thickness of the dielectric layers), cuts the created electrostatic force for a given voltage a factor of 4. The curves in either type of design create greater distances between the diaphragm/membrane with electrode and the fixed electrodes that must be compensated for with voltage to achieve a given actuation force.
Another important problem with all touch-mode electrostatically actuated devices is that the high electric field within the dielectric can be high enough to cause arcing across the dielectric material over time, and that the dielectric degrades with time, rendering the device useless. In addition, even if the applied voltage is kept low enough that direct arcing failures do not occur, electrical charges can move into the bulk of the dielectric and/or onto the surfaces at the interface of the touch-mode electrodes, which then diminish the electric field providing the actuation force, reducing both the pressure that can be switched and/or increasing the time required for switching.
More problematic is that the charges trapped in the bulk and/or on the surfaces can create an electrostatic sticking force that can prevent the device from working at all. The phenomena of charge trapping has been recognized in the art, but comprehensive solutions that limit the trapping of charges in thin dielectric layers that permit a small gap are lacking. Trapped charges accumulate in time, which can significantly shorten device lifetime and reliability.
Many applications would benefit from a rapid action touch mode capacitance microvalve. Some problems have been individually addressed in prior proposed microvalves, but the present inventors recognize that a need exists for high performing microvalves that operate under significant pressures.